Equipment that uses batteries as its driving power source, such as electric bicycles, electric motorcycles and electric vehicles, employs a battery pack in which large-capacity secondary batteries are housed. Lithium ion secondary batteries, which are large in both volumetric energy density and mass energy density, are suitable as batteries for driving power sources.
Among those known are a columnar lithium ion secondary battery in which strip-like positive and negative electrodes are stacked via separators before being wound, and a flat lithium ion secondary battery in which thin-plate positive and negative electrodes are stacked via separators.
Among them, flat batteries are especially suitable for power-source batteries of power motors and others since the capacity per unit battery can be easily raised by increasing the area or number of positive and negative electrodes stacked.
The unit batteries of a flat lithium ion secondary battery have their battery elements covered with the film-like exterior material. In this manner, the high energy density of the lithium ion secondary battery is effectively used.
In the lithium ion secondary battery, in order to prevent metal lithium from being deposited in dendrites due to the concentration of current into a corner portion of a negative electrode during charging, the dimensions of positive electrodes, separators and negative electrodes are appropriately determined. That is, as for the stacking-position relationship between the coating section of the positive electrode active material, the coating section of the negative electrode active material and the separators, the dimensions of the separators are the largest. Moreover, the outer periphery of the separators are positioned outside the outer peripheries of the coating section of the positive electrode active material and of the coating section of the negative electrode active material. In addition, the dimensions of the coating section of the positive electrode active material are the smallest, and the outer periphery of the coating section of the positive electrode active material is positioned closer to the inner side than the outer peripheries of the separators and of the coating section of the negative electrode active material.
When used at high temperature, the separators would significantly contract thermally. There is one proposed technique by which the shape of separators are set larger in advance with a heat shrinkage rate in mind so that the thermal contraction of the separators does not lead to the positive and negative electrodes being short-circuited (Refer to Patent Documents 1 and 2, for example).
The positive and negative electrodes of the lithium ion secondary batteries are produced by applying and drying slurry, which is made by mixing active material and particles of a conductive auxiliary agent with a binder. Accordingly, there are concerns that, when being subjected to vibration or shock as a result of the battery falling or any other problem, the battery could see the active material come off or the electrodes broken or any other trouble.
Moreover, in the stack of thin-plate positive and negative electrodes, the outer periphery of the thin-plate negative electrodes are situated closer to the exterior body than the outer periphery of the thin-plate positive electrodes. Therefore, the thin-plate negative electrodes are more affected by external shock as a result of contact with the exterior body.
Meanwhile, as disclosed in Patent Documents 1 and 2, if the outer periphery of the separators are positioned outside the portions excluding the non-coating sections of the active material, the separators, which are exposed from the stack more than the electrodes, would serve as a cushioning member for weak shock. Such a configuration is expected to reduce such troubles as the breaking of the electrodes or the dropping of the active material.
There is also a proposed battery: Instead of providing or inserting separators between positive and negative electrodes, the positive electrodes are contained in bag-shape separators; and the bag-shape separators containing the positive electrodes, and the negative electrodes are stacked alternately (Refer to Patent Documents 3 or 4, for example).
The insertion of the positive electrodes into the bag-shape separators prevents the active material from adhering to the counter electrode, with the help of the bag-shape separators, even if the positive electrode active material or the negative electrode active material comes off from a current collector. This prevents the quality deteriorating.
When the bag-shape separators are produced, wrinkles will likely appear due to thermal contraction. In Patent Document 3, the heat sealing of the separators is conducted at certain intervals to prevent such wrinkles appearing.